Rock anchors using bolting principles are--according to their purpose--inserted into the rock in different lengths and have different diameters. They bear different load forces in underground structures by insertion in the rock surrounding a mine shaft or gallery to increase the load bearing strength of the rock.
The increase in load bearing strength of the rock mantel surrounding the excavated gallery or tunnel can result because the layered, slightly stable rock layers are "suspended" or fixed in compacted layers located over them.
Stabilization also results from the frictional locking or contact between the neighboring layers and thus the rigidity of the anchored total structure is increased by the load force of the anchor (compression or by load forces built up by rock movement) so that the rock motion is opposed directly by the strong resistance of the rock anchor or the anchor--comparable to a concrete reinforcement--increases the binding strength of the rock.
To fulfill this function the rock anchor can be equipped usually with special devices at its end located in the deepest portion of the hole in the rock. These special devices provide an adhesive or binding action in the ground by spreading or attaching to the hole wall.
Advantageously a multicomponent adhesive (e.g. an epoxy resin) can be provided at the hole wall. In one case the rock anchor is provided with adhesive along its entire length in the hole. At its free end projecting from the hole, the rock anchor is provided with a screw and nut or with a screw head mounted on the anchor by which a supporting member bearing the anchoring force is formed by an anchoring plate pressed against the free rock surface at which the rock-bolt hole opens.
The anchor structure is mostly used in tunnel structures to keep the rock around the subterranean chamber rigid prior to building the final support structure (chiefly a single or multi-layered lining concrete) and to increase the self supporting properties of the surrounding rock after the final structure is constructed. These functions can be assisted in the tunnel structure by selecting an optimal cross section for the effectiveness of the anchoring structure.
In subterranean excavating processes, such as mining, e.g. in local supporting pillar mining, in which only a portion of the minerals are mined and columns or pillars for support of the overlying layers remain in the deposit between the chambers originating from the mining, the cross section of the gallery chiefly results from the character of the mined bed alone so that the function of the anchoring structure in this case is not assisted by an optimized cross sectional shape. The anchor structure has been extraordinarily effective in bituminous coal mining in the United States, Canada, Australia and South Africa.
In the mining process in which columns of the mined minerals remain between the mine structure for support of the overlying rock layers in the deposit, the rock anchor is used to absorb only comparatively small rock motions which result from the elastic back compression of the rock surrounding the chamber and from the plastic motions which are ascribed to the compression distribution surrounding the rock chamber. With the complete mining of the deposit which is the predominate mining method in European coal mining considerably larger rock motions arise which are a result of the substantially stronger disturbance of rock surrounding the chamber. In long-wall mining, which is in fact the predominate mining process, mining must be effected in zones in which as a result of the additional pressure extraordinarily high forces occur. The additional pressure amounts in these zones chiefly to a multiplication or amplification of the superposition pressure.
These pressure manifestations which complicate tunnel building and local supporting pillar mining are the basis for using the anchor structure in European and particularly German coal mining only under particularly good neighboring ground conditions, i.e. with comparatively good stability or rigidity of the surrounding ground. There have also been many attempts to use flexible rock anchors to correspondingly improve the situation when there are higher rock motions as a result of complete mining of the deposit. For years, however, no breakthrough or other success has occurred. The use of the exceptional rock anchor in German coal mining has thus been comparatively limited as has been mentioned previously.
The rock anchor structure most widely used in coal mining (both in local supporting pillar mining and with complete mining of the deposit) comprises an anchoring rod or bolt whose surface is shaped or profiled to improve the action of the adhesive. Since the anchor is exposed to the rock motion only a material can be used which has a sufficient stress resistance or flexibility up to its elastic limit for breaking. With previously used anchor cross sections the highest allowed load is limited to a comparatively small value.
On account of the considerably larger rock motions in complete mining of the deposit as opposed to local supporting pillar mining, rock anchors are used which comprise materials with substantially higher elasticity. Apart from the fact that with these anchors the highest allowed load is reduced a considerable amount by comparison with the previously used anchor cross sections, these anchors are too expensive because of excessive material cost.
Attempts have been made to use rock anchors with two diameters in which the larger diameter body is reduced by a drawing die. This gives a certain flexibility under load. Apart from the fact that these anchors are much more expensive than the standard anchor the underground trials or attempted uses have led up to now to no satisfactory results.
It has been proposed to use elongated reinforced concrete bodies as anchors to permit higher supporting force to be developed with the previously used hole diameter as a result of the use of materials of higher strength. These anchors are provided with a flexible element which works according to the friction principle (analogous to the known friction prop). They have the advantage in contrast to all other structures used in coal mining that the anchor rods are flexible and can bend around corners. This kind of insertion is particularly important in narrow curved chambers which should be safely secured with anchors whose length exceeds the dimensions of the chamber. Furthermore they provide considerable technical and economical advantages in gallery digging or tunnel construction because the anchor can be inserted above the tunnel or gallery digging machine and the idle time is usually considerably reduced. That idle time is presently almost 50% of the available running time of the machine.